In deployments of wireless mobile networks, and in particular in 3GPP's Long Term Evolution (LTE), a set of base stations or eNB's can make use of the same radio resources in the time and frequency domain, creating interference to terminals (UE's) located in the border coverage areas between them. This situation is particularly severe in the case of the so called heterogeneous networks, where a layer of high power macro eNB's is overlaid with layers of lower power eNB's that are deployed in the same area. Such deployments can achieve significantly improved overall capacity, although cell-edge performance can be degraded due to intra-carrier interference. Interference reduction in these scenarios is the object of techniques known as Inter Cell Interference Coordination (ICIC).
A procedure for interference reduction in wireless communications where the same radio resources are used in two different base stations is the so called Time Domain Multiplexing ICIC. In this approach the transmissions from an aggressor eNB inflicting interference onto other are periodically muted for entire subframes, so that the victim eNB have a chance to serve their UE's in these subframes. This muting is not complete, as certain signals like common reference symbols, synchronization signals or a broadcast channel have to be transmitted even in the muted subframes. Subframes that are muted are called almost blank subframes (ABS).
ABS muting patterns are configured semi-statically and signalled between eNB's through the X2 interface. Signalling is done by means of bit maps of length 40 or 70, representing the ABS pattern over four frames for FDD mode, and two to seven frames for TDD. The ABS pattern can be configured by the network in a static way, or it is possible to apply self-optimizing networks (SON) function for optimizing the muting pattern according to some target criterion like load balancing and taking into account the traffic in both the aggressor eNB and the victim eNB's.
ABS muting pattern is communicated from the aggressor eNB to the victim eNB by means of an “ABS Information” Information Element (ABS Information IE), as it is described in [1]. The ABS Information IE includes the “ABS Pattern Info”, which is a 40 bit string. This 40 bit string represents 40 subframes in 4 consecutive frames, the first bit representing the first subframe of the 40 subframes, and the last bit the last subframe of the 40 subframes. In this 40 bit string, a bit equal to 1 represents an ABS subframe, and a bit equal to 0 an ordinary non-ABS subframe. The ABS pattern starts in the System Frame Number 0, and is repeated continuously every 4 frames (40 ms). The System Frame Number is increased every 10 ms (every frame) and runs from System Frame Number 0 to System Frame Number 1023, therefore an ABS pattern remains active for at least 1024 frames, or 10.24 s. The aggressor eNB can send a new ABS Information IE to the victim eNB with a new ABS Pattern Info, which will come into effect when the System Frame Number is again 0. FIG. 1 shows an ABS pattern and its representation as a 40 bit string.
The ABS Information IE is part of the Load Information message, which is sent through the X2 interface, as it is described in [2][2]. The purpose of the Load Information message is to transfer load and interference co-ordination information between eNB's controlling intra-frequency neighbouring cells.
TABLE 1Load Information messageIE typeandIE/Group NamereferenceSemantics descriptionMessage TypeCell Information >Cell Information Item >>Cell IDIdentification of thesource cell >>UL Interference Overload Indication >>UL High Interference Information  >>>Target Cell ID  >>>UL High  Interference  Indication >>Relative Narrowband Tx Power (RNTP) >>ABS InformationInformation Element that containsthe ABS pattern information >>Invoke Indication
If the ABS Information IE is included in the Load Information message, the ABS Pattern Info IE indicates the subframes designated as almost blank subframes by the sending (aggressor) eNB for the purpose of interference coordination. The receiving (victim) eNB may take such information into consideration when scheduling UE's. The structure of the ABS Information IE is as follows.
TABLE 2ABS Information IEIE type andIE/Group NamereferenceSemantics descriptionABS Information——>FDD—— >>ABS PatternBIT STRINGEach position in the bitmap represents a DL subframe, for which Info(SIZE(40))value “1” indicates ‘ABS’ and value “0” indicates ‘non ABS’.The first position of the ABS pattern corresponds to subframe 0 in aradio frame where SFN = 0. The ABS pattern is continuouslyrepeated in all radio frames.The maximum number of subframes is 40. >>Number OfENUMERATEDP (number of antenna ports for cell-specific reference signals) Cell-specific(1, 2, 4, . . . )defined in TS 36.211 [10] Antenna Ports >>MeasurementBIT STRINGIndicates a subset of the ABS Pattern Info above, and is used to Subset(SIZE(40))configure specific measurements towards the UE.>TDD—— >>ABS PatternBIT STRINGEach position in the bitmap represents a DL subframe for which Info(1 . . . 70, . . . )value “1” indicates ‘ABS’ and value “0” indicates ‘non ABS’.The maximum number of subframes depends on UL/DL subframeconfiguration.The maximum number of subframes is 20 for UL/DL subframeconfiguration 1~5; 60 for UL/DL subframe configuration 6; 70 forUL/DL subframe configuration 0.UL/DL subframe configuration defined in [3]The first position of the ABS pattern corresponds to subframe 0 in aradio frame where SFN = 0. The ABS pattern is continuouslyrepeated in all radio frames, and restarted each time SFN = 0. >>Number OfENUMERATEDP (number of antenna ports for cell-specific reference signals) Cell-specific(1, 2, 4, . . . )defined in TS 36.211 [10] Antenna Ports >>MeasurementBIT STRINGIndicates a subset of the ABS Pattern Info above, and is used to Subset(1 . . . 70, . . . )configure specific measurements towards the UE>ABS InactiveNULLIndicates that interference coordination by means of almost blanksub frames is not active
If the Invoke Indication IE is included in the Load Information message, it indicates which type of information the sending eNB would like the receiving eNB to send back. If the sending eNB is the victim eNB, the victim eNB can make use of the Invoke Indication IE to request the activation of ABS subframes in the aggressor eNB. In this case the Invoke Indication IE is set to “ABS Information”, and it indicates that the sending (victim) eNB would like the receiving (aggressor) eNB to initiate the Load Indication message, with the Load Information message containing the ABS Information IE indicating the ABS pattern.
On the other hand, the aggressor eNB can request to the victim eNB to report the status of the ABS subframes usage at the victim eNB by means of a Resource Status Request message on the X2 interface. The victim eNB will report back by means of the Resource Status Update message [4], which includes the ABS Status IE. The ABS Status IE is used to aid the aggressor eNB which is creating the ABS pattern to evaluate the need for modification of the ABS pattern. The ABS Status IE is defined in [5] and indicates a percentage of used ABS resources in the victim eNB.
The standard procedure for reporting the status of radio resources usage between eNB's through the X2 interface is as follows. The process starts by means of a Resource Status Request message [6], which is sent by an eNB to a neighbouring eNB to initiate the requested measurement according to the parameters given in the message, and the neighbouring eNB answers with a Resource Status Response message, which is sent to indicate that the requested measurement, for all or for a subset of the measurement objects included in the measurement is successfully initiated. Then, the neighbouring eNB will send Resource Status Update messages to report the results of the requested measurements [4][4].
The contents of the Resource Status Request message, where eNB1 is the sending eNB and eNB2 the neighbouring eNB, as is showed in table 3.
TABLE 3Resource Status Request messageIE type andIE/Group NamereferenceSemantics descriptionMessage TypeeNB1INTEGERAllocated by eNB1Measurement ID(1 . . . 4095, . . . )eNB2INTEGERAllocated by eNB2Measurement ID(1 . . . 4095, . . . )RegistrationENUMERATED(start,A value set to “stop”, indicates a request to stop all cellsRequeststop, . . . )measurements.ReportBITSTRINGEach position in the bitmap indicates measurement objectCharacteristics(SIZE(32))the eNB2 is requested to report.First Bit = PRB Periodic,Second Bit = TNL load Ind Periodic,Third Bit = HW Load Ind Periodic,Fourth Bit = Composite Available Capacity Periodic,Fifth Bit = ABS Status Periodic.Other bits shall be ignored by the eNB2Cell To ReportCell ID list for which measurement is needed >Cell To Report Item >>Cell IDECGIReportingENUMERATED(1000 ms,Periodicity2000 ms,5000 ms, 10000 ms, . . . )Partial SuccessENUMERATED(partialIncluded if partial success is allowed.Indicatorsuccess allowed, . . . )
Regarding the contents of the Resource Status Response message, it is showed in Table 4.
TABLE 4Resource Status Response messageIE type andSemanticsIE/Group NamereferencedescriptionMessage TypeeNB1 Measurement IDINTEGERAllocated by eNB1(1 . . . 4095, . . . )eNB2 Measurement IDINTEGERAllocated by eNB2(1 . . . 4095, . . . )Cell Measurement Result >Cell Measurement Result Item >>Cell IDECGI >>Hardware Load Indicator >>S1 TNL Load Indicator >>Radio Resource Status >>Composite Available Capacity Group >>ABS Status
A problem with the existing solutions is that in the current ABS implementation, the aggressor eNB commits to not using the ABS subframes that it has reported in the ABS Information IE as ABS subframes until a new ABS Information IE is sent and a new ABS pattern starts at System Frame Number o. Therefore, a given amount of radio resources at the aggressor eNB are reserved for ABS subframes for a period of 10.24 seconds and the aggressor eNB will not use them although it could require serving traffic higher than expected. The result of the current ABS implementation is that an aggressor eNB could require serving a peak traffic demand and not having any radio resources available, although some of the subframes labelled as ABS subframe perhaps are not used by the victim eNB, thus making an inefficient use of the radio interface.
Another related disadvantage of the current ABS solution is that it limits the flexibility of the ABS subframes to distribute the traffic load between the aggressor eNB and the victim eNB, because if the aggressor eNB seeks to implement a high traffic load diversion to the victim eNB, by means of reserving many subframes to the ABS pattern, it runs the risk of not being able to support unexpected data traffic demands in the aggressor eNB. FIG. 2 represents the situation where some ABS subframes remain unused by both the aggressor eNB and the victim eNB.
On the other hand, the ABS Status IE only reports a percentage of the ABS subframes used by a victim eNB in the past, but does not include an indication of the expected ABS subframes to be used in the future and its related probability.
Another problem of the current ABS implementation is that it is not efficient when two or more victim eNB's are interfered by the aggressor eNB. The aggressor eNB will define a single ABS pattern, and every victim eNB will make use of the ABS subframes without coordination of the ABS subframes selected by the other victim eNB's. As a result, two neighbouring victim eNB's can select the same ABS subframes for serving their own UE's, causing interfering to each other. FIG. 3 represents the situation where some ABS subframes are used by two neighbouring victim eNB's and thus potentially producing interference to each other.
On the other hand, two non-neighbouring victim eNB's can select different ABS subframes, when they could select the same ABS subframes without interfering each other and thus leaving more possibly unused ABS subframes for the aggressor eNB usage, as it is represented in FIG. 4.